1. Field of the Invention
This invention relates to high electron mobility transistor (HEMT) devices and method of making the same.
2. Description of the Related Art
GaN based materials have physical and electronic properties that make them attractive for high temperature, high power and high frequency devices. Wide bandgap semiconductors (GaN and SiC) have inherently lower thermal carrier generation rates and higher breakdown fields compared to Si and GaAs, as shown in Table 1 below.
TABLE 1Properties of candidate materials for high power, high temperature,high frequency electronic devicesMaterial PropertySiGaAs4H-SiCGaNBandgap (eV)1.11.43.33.4Breakdown field (105 V/cm)243030?  Electron mobility (cm2/Vs)14008500800900a, 2000bMaximum velocity (107 cm/s)1223  Thermal conductivity1.50.54.91.3(W/cm K)afor n = 5E16 cm−3;bfor an AlGaN/GaN structure
GaN has additional advantages including a high (>800 cm2/Vs) electron mobility and a high (>107 cm/sec) electron velocity. Furthermore, high electron mobility transistors (HEMTs) which offer higher mobilities, better charge confinement and higher breakdown voltages can be fabricated in the AlGaN/GaN materials system. Room temperature radio frequency (8-10 GHz) output powers on the order of 6-8 W/mm are theoretically possible in the AlGaN/GaN materials system and power densities as high as 6.8 W/mm have recently been reported (S. T. Sheppard, et al., 56th Device Research Conference, Charlottesville, Va., Jun. 22-24, 1998).
While promising output powers have been reported in AlGaN/GaN HEMTs, materials-related issues continue to limit device performance. Persistent photoconductivity (PPC) and drain I-V collapse have been reported in AlGaN alloys (M. D. McCluskey, N. M. Johnson, C. G Van De Walle, D. P. Bour, M. Kneissl and W. Walukiewicz, Mat. Res. Soc. Symp. Proc. 521 (1998), p. 531) and AlGaN/GaN heterostructures (J. Z. Li, J. Y. Lin, H. X. Jiang, M. A. Khan and Q. Chen, J. Appl. Phys. 82 (1997) 1227). These effects arise from carrier trapping and generation from deep levels in the material and can lead to poor high frequency performance, decreased drain currents and reduced output powers in a HEMT. PPC and current collapse in GaAs-based HEMTs have been attributed to defect-donor complexes (DX centers) in AlxGa1-xAs when x>0.20. Evidence for oxygen DX-centers in Al-rich AlxGa1-xN (x>0.27) has recently been reported (M. D. McCluskey, et al., ibid.). High Al content AlGaN layers (x>0.20) are commonly used to achieve high sheet densities in AlGaN/GaN HEMT structures via piezoelectric-induced doping as shown by the data in FIG. 1, which is a plot of sheet density as a function of percent aluminum composition in undoped 23 nanometer AlGaN/GaN heterostructures.
In order to further improve the performance of III-V nitride HEMTs, methods must be identified to reduce or eliminate the deleterious effects of deep level defects that result from the use of high Al composition layers.